config: support loading from kubeconfig string

[colemickens]

Fixes #1028.

I've manually verified this by building the kubernetes-plugin for Jenkins with my modified library and was able to connect to a cluster via a kubeconfig provided as a secretText by utilizing the fromKubeconfig static method added here.

My intent was to reduce code duplication, while still preserving the existing code path with tryKubeConfig and with the relative->absolute path rewriting in kubeconfig.

[rohanKanojia]

ok to test

[oscerd]

ok to test

[piyush-garg]

@colemickens Can you please add the changes to CHANGELOG.md

[colemickens]

@piyush1594 I have pushed a new commit with an updated CHANGELOG.md.

[oscerd]

[merge]

[colemickens]

Sorry to ask, but is there any guidance on when releases are cut? I see that it's been almost 3 months since the last release. I'm anxious to use this new functionality in a release package.

Thanks!